This invention relates to a modulation system for use in carrying out linear modulation in a radio communication system for digital signal transmission or analog signal transmission. It is to be noted throughout the instant specification that quadrature amplitude-phase modulation is exemplified as the linear modulation employed although this invention is not restricted to the quadrature amplitude-phase modulation.
Recently, deficiency or lack of a radio wave resource, namely, a radio frequency band, has been pointed out and awakens a new interest in narrowing radio frequency bandwidth for each radio channel so as to expedite effective use of the radio frequency band. In the light of narrowing the radio frequency bandwidth for each radio channel, linear modulation is superior to nonlinear modulation, such as frequency modulation, because the latter brings about inevitable expansion of the radio frequency bandwidth for each radio channel.
In general, a modulation system for such linear modulation is for use in combination with an amplifier, such as a solid state power amplifier or a traveling wave tube (TWT) amplifier, which inevitably has nonlinearities in phase and amplitude. The nonlinearities of the amplifier come from nonlinear AM/AM and AM/PM conversion characteristics and give rise to degradation of a transmission spectrum of a transmission signal produced by the amplifier and to deterioration of a reception characteristic of a receiver for receiving the transmission signal. In addition, the nonlinearities cause an intermodulation component to occur and bring about interference between adjacent ones of the radio channels.
As will later be described in conjunction with a figure of the accompanying drawing, an adaptive predistortion circuit is disclosed by Davis et al in U.S. Pat. No. 4,291,277 in order to compensate for the nonlinearities of the amplifier. The adaptive predistortion circuit is placed prior to the amplifier and is supplied with a sequence of digital signals at a symbol rate predetermined for the digital signals and with a sequence of additional digital signals derived from an output signal of the amplifier. In short, the adaptive predistortion circuit is operable to supply the amplifier with a modulated analog signal subjected to predistortion compensating for the nonlinearities. Such predistortion is adaptively carried out with reference to the additional digital signal sequence at the symbol rate.
More particularly, each of the digital signals is delivered as an address signal to a random access memory (RAM) and a read-only memory (ROM). The read-only memory is loaded with a plurality of invariable reference signals while the random access memory is variably loaded with predistorted signals.
Under the circumstances, one of the invariable reference signals is read out of the read-only memory in response to one of the digital signals and is compared with a received one of the additional digital signals to calculate a difference between the one invariable reference signal and the received additional digital signal. On the other hand, one of the predistorted signals is read out of the random access memory in response to the above-mentioned one of the digital signals and is modulated into a modulated analog signal through a modulator to be sent to the amplifier.
Simultaneously, the above-described one of the predistorted signals is adaptively modified into a modified signal with reference to the above-mentioned difference. The modified signal is substituted for the above-described one of the predistorted signals in the random access memory. Thus, the predistorted signals are adaptively updated or modified in consideration of the reference signals and the additional digital signals.
It should be noted that modification of the predistorted signals is carried out at the symbol rate of the digital signal sequence. In other words, the nonlinearities of the amplifier are compensated at every symbol instant of time appearing at the symbol rate and are not compensated during an intermediate time interval between adjacent ones of the symbol instants, both exclusive. Accordingly, the transmission spectrum is not improved during the intermediate time interval and intermodulation is liable to occur during the intermediate time interval.
Moreover, no consideration is paid to analog transmission about application of the modulation system because the teaching of Davis et al is not directed to a relationship between the symbol rate and a sampling rate necessary for sampling an analog signal, such as an audio signal.